1. Field of the Invention
This invention relates to a locomotion control system for a legged mobile robot, more particularly to such a system which feedback-compensates the discrepancy between the robot's rigidity assumed in a model and the actual robot's rigidity so as to increase the apparent or virtual rigidity and enable the robot to be treated in terms of an ideally rigid model.
2. Description of the Prior Art
Legged mobile robots are known to the art. See the teaching of Japanese Laid-Open Patent Publication No. 62(1987)-97,006, for example.
To be able to walk stably, a legged mobile robot is required to satisfy prescribed dynamic stability conditions. These conditions can be met either by solving dynamic problems in real time as they are encountered during locomotion or by solving the problems in advance. For realizing either method, the robot has to be mathematically modeled. While the model should preferably approximate the actual robot with a high degree of accuracy, this is hard to achieve in actual practice. Since numerous factors involved are difficult to model and because of various other restrictions, such as the need to shorten processing time or reduce the labor required for creating the model, the model used is invariably only an approximate one.
It is a characteristics of the legged mobile robot that the legs have to be swung at high speed during walking, while, at the same time, the joints and links of the legs are subject to heavy loads (bending moments). Because of this, it is preferable for the robot's legs to be light in weight and low in inertia (inertial force) so as to reduce the amount of energy required for swinging the legs and also to prevent stability degradation owing to the reaction forces produced by the swinging of the legs. If, however, the mathematical model of the robot assumes high rigidity or if it does not take the robot's elastic elements into account, the actual robot rigidity also has to be high. Thus the robot has to be light in weight and low in inertia, in addition to being rigid. It is difficult to satisfy both of these requirements. A legged mobile robot includes joints and other moving parts that cannot easily be made high in rigidity. It also uses force sensors of the type which detect strain and various other elements which cannot easily be increased in rigidity. As a practical matter, therefore, it is next to impossible to achieve the required weight and inertia reduction while simultaneously increasing rigidity to the point where the elastic elements of the robot can be ignored. Because of this, any attempt to satisfy the dynamic stability requirements for walking by the use of a rigid model for a robot that includes elastic elements will be prevented from realizing the desired walking stability by the approximation errors between the actual robot and the model.